Electrodeposition of zinc



able to good performance.

Patented Jan. 1, 1946 ELECTRODEPOSITION F ZINC RlchardM. Wick, Allentown, 'Pa.,"ass ign'or to Bethlehem Steel Company, a corporation of Pennsylvania 'Appllcation November 12, 1941, Serial No. 418,861

9 Claims. (Cl. 204-55) My invention relates to the electrodeposition of zinc. It pertains more particularly to zinc plating, although it may be utilized wherever it is desired to electrodeposit zinc, as in the recovery of zinc from solutions of its compounds.

My invention comprises methods of electrodepositing zinc from solutions of its compounds, the most significant feature of which is the use of relatively high temperatures of the electrolytes employed. It is applicable to electrodeposition of zinc from electrolytes containing free acid, and is particularly adapted to electrodeposltion from highly acid electrolytes.

It has long been believed by those skilled in the art that zinc could be deposited eificiently only from acid solutions which were kept relatively cool, and since, with comparatively high current densities, the temperatures of the solutions tend to rise substantially above what has been deemed a suitably low temperature, it has been the-practice to utilize means for artificially cooling. In an article by Tainton and Leyson in the Transactions of The American Institute of Mining and Metallurgical Engineers, vol.

LE, 1924, the following is stated: As hydrogen over-voltage, and consequently current emciency, falls with increasing temperature, it is necessary to keep the cell temperatures within certain limits. Ralston says: The conclusion is inevitable that low temperature is most favorable to the operation of the zinc cell. Experience has indicated temperatures of 75-100 F. as most favor- Except in cold weather, the radiation from the cells does not dissipate suflicient heat to keep temperatures down to this point; consequently the practice of cooling the electrolyte in hot weather by means of lead pipes containing cooling water has been universally adopted. (See pp. 516 and 517.)

I have discovered that for high current densimal, in that each expresses the relationship between current density and eihciency for a particular temperature of the electrolyte, curve A" representing such relationship at 30 C., and

curve B representing it at 75 C.

These curves have been plotted upon data obtained when using the same composition of electrolyte for each curve, namely, an electrolyte containing 58 grams of zinc per liter, and 220 grams of sulfuric acid per liter. It is to be understood that the curves would be somewhat different if the composition of the electrolyte or the temperature thereof had been different. These curves, however, adequately represent the difference in effects resulting from using a relatively high temperature electrolyte as compared with the effects from using. an electrolyte with a relatively low temperature.

Curve A, indicating the relationship of current density and efliciency, when the electrolyte is maintained at 30 C., and therefore representative of those conditions under the practice prior to my invention, shows that, with increased ourties in the electrodeposition of zinc I can obtain higher electrochemical efliciencies and other advantages by using just what the prior art has avoided, namely, relatively high temperatures of the electrolytes.

In the drawing, which forms a part of this specification, I show curves to represent graphically how the use of higher electrolyte temperatures gives me certain distinct advantages over the prior practice. On the axis of aJbsc-issas OK, I designate current densities expressed as amperes per square foot of cathode surface, and on the axis of ordinates OY, I designate current eiiiclencies expressed in percentages of perfect eil'iciency. Each of the two curves shown is isotherrent density the efficiency rises rapidly to a maximum, continues at this maximum over a comparatively small range of current densities, and then falls rapidly. With the particular electrolyte composition used, and when maintaining it' at 30 C., the efficiency rises to a maximum of somewhat less than with a current density of 3'75 amperes per square foot of cathode surface, and begins to fall when the current density reaches somewhat over 500 amperes per square foot of cathode surface, the drop in emcierlcy trolyte, when this electrolyte is maintained at 75 C., and representative of my invention, shows that with increasing current density the efliciency rises quickly to a maximum of over with a current density of somewhat over 1000 amperes per square foot of cathode surface, and with further increase of current density the eflieieney diminishes at a very slow rate, this portion of the curve being a straight line very slightly inclined to the horizontal. Even when the efliciency" is carried to 3000 amperes per square foot of cathode surface the efliciency is still about 92%, this being but a slight drop from the maximum efiiciency obtainable with this particular electrolytic composition when used at 75 C.

Comparing curves A and B by examining them from left to right, it will be seen that there are three different phases, .namely, an upwardly inclined, relatively straight, portion, succeeded by a more or less gradually curving portion, and then a relatively straight line portion extending to the right. In the drawing, the point which marks the approximate termination of the curve portion and the beginning of the second mentioned relatively straight line portion is indicated by m for curve A and by n for curve B.

The straight line portions of curves A and B, v

beginning at points 1n and 11, respectively, are of considerable significance because it is under the conditions represented by the straight line portions that the most satisfactory kind of zinc plate is produced. When working under conditions represented by those portions of the curves A and B to the left of points m and u, there is a strong tendency for the occurrence of bare spots during plating on ferrous material, this tendency increasing the farther one goes to the left of these points.

It will be readily apparent that point n on curve B is considerably higher than point 122 on curve A and that it is located considerably farther to the right. It will also be noted that the straight line portion of curve B, extending to the right of point 12, has but a slight downward slope from the horizontal while the corresponding straight line portion of curve A inclines downwardly at a considerable angle to the horizontal. Interpreting this relationship of the two curves, in terms of operation of the two processes, it is obvious that the emciency of operation for the upper "straight line" conditions at 75 C. is much higher than when operating at 30 C. and that this difference in efliciency between the two temperature.

conditions of operation increases when the current densities are increased, 1. e., the higher the current density chosen for electrodeposition, the greater the advantage in efilciency for the use of the higher temperature operation, because, while the straight line portion of curve A declines rapidly the corresponding portion of curve B drops very slowly. The fact of n on curve B being considerably to the right of m on curve A is significant in that it indicates that the most effective operating conditions of the higher temperature processes are secured at much higher current densities than with the lower temperature operation. In other words, my process involves not only an increase in temperature of the electrolyte -but the use of higher current densities. As already indicated earlier in this specification, the choice of different temperatures and compositions of electrolytes will produce curves different from those shown in the drawing, but, in all cases, when using the higher temperatures involved in my invention, higher current densities go along wit the higher temperatures. 4

The advantages to be obtained by my invention will be readily apparent to those skilled in the art. In the first place substantially higher efliciencies are available than are possible with the previous practice of using comparatively low temperature electrolytes.

Probably the greatest advantage of my invention is that my process permitsthe use of an almost unlimited range of high current densities while still retaining high efllciencies. From curve "A, representative of the prior practice, it is evident that, even with but moderately high current densities, the efficiencies become rather low.

At 1000 amperes per square foot of cathode surface, the efllciency is but 80%, rapidly dropping to low values if the current density is further increased. With my process, current densities 5 from moderately high to very high may be used while retaining high efficiency. In the example of my process, illustrated by curve B, from a current density of 750 amperes per square foot of cathode surface to current densities of the order of 5000 amperes per square foot of cathode surface can be used while retaining efficiencies in the vicinity of 90% or greater.

My process has great utility in plating operations, particularly in continuous processes for zinc plating iron and steel material, such as sheet or wire. The continuous plating of wire furnishes a good example of some of the advantages of my invention. As well known, in such processes a plurality of wires are fed continuously through a plating tank containing the electrolyte. The speed of movement of the wire through the plating tank or tanks determines the capacity of the particular plating equipment. Now, the permissible operating speed of the wire varies directly with the product of the current density and efliciency. This speed may be expressed by means of a formula as follows:

where It indicates a constant; E, the efliciency; J, the current density; L, the length of the operative part of the tank or tanks; and W, the weight of zinc to be deposited. Obviously, for a particular equipment the factor L is a constant; and for a particular weight of zinc to be deposited W is also a constant. The speed, therefore, of the wire through the equipment depends directly upon the product of the efficiency and current 4" density.

The importance of my invention under the conditions just outlined will be readily apparent when one compares curves A and B of the drawing. If one seeks to increase the speed of the a wire through the tanks, to thereby increase the capacity under the conditions or curve A, he soon reaches a point beyond which further increase of current density is accompanied by a fairly rapid drop in current efllciency; and, since the speed of the wire through the electrolyte is a function of the product of both current density and efficiency. the effect of the increase in current density in increasing the capacity of the equipment, is to a. considerable degree reduced by 65 the drop in efficiency. Not only this, but the decided drop in efliciency necessarily means a substantial loss of eflective current and thus an increase in cost of operation in addition to a corresponding lowering of the output of product possible from the installation. Itis therefore but natural that zinc plating. as practiced prior to my invention, has :been decidedly limited in the use of any but moderately high current densities.

In my invention, on the other hand, as illustrated in curve B, current densities may be increased to any desired practical extent without substantial losses in efllciencies. Thus the sp ed of the wire, and therefore the production capacity oi the equipment maybe greatly increased and without substantial losses in current. It will thus be seen that, by means oi my invention, it is possible not only to deposit zinc at a lower operating cost but it is also possible, for a given production. to greatly lessen the original cost for equipment. since, obviously, increased capacity for production requires less equipment for a given output.

As a further example of the advantages to be secured from my invention, I call attention to a condition which is frequently met with in certain zinc plating operations. In wire coatin for instance, it is often necessary in practice to plate wire of different diameters in the same apparatus; in other words, it is necessary to use the same equipment to coat the various sizes of wire called for by the trade. It is also important from a practical standpoint that the current through the electrolytic cell be maintained constant and at a maximum. It will be evident that when there is a change from one size of wire to another the current density will change. Supposing the equipment is being used to coat a particular size wire, under conditions to give maximum eillciency, and supposing the practice is that illustrated by curve A," it will be evident that if the size of the wire is changed'to that of a smaller diameter, the current density will rise and the efficiency drop. With my process, however, if the operation is being conducted on the nearly horizontal portion of curve B, the change in size of wire, resulting in a change of current density, will cause no substantial alteration of the emciency.

Earlier in this specification, I have pointed out that the most satisfactory zinc plate is secured when operating under conditions corresponding with the upper straight line portions of the curves, and that when operating to the left of the straight line portions there. tends to be more or less bare spots, this tendency increasing the farther to the left one goes. Prior to my invention this fact led to the practice of using a strike coating. To secure a good coating the material to be plated has been first subjected to electrolysis with a current density sufliciently high to bring the operation within the straight line portion, such as indicated, for example, in curve A of the drawing. As is obvious from curve A, this strike treatment would not be at as high an efiiciency as desirable because of the fact that the straight line of the prior art practice slopes rapidly downwardly. Consequently, after giving the article to be treated an exceedingly thin coating of zinc ("strike coating) with a high current density, the coating operation would be completed by subjecting to electrolysis at lesser current densities, thereby to work at higher efliciencies. By means of my invention I am able to dispense with this two-stage type of treatment inasmuch as the entire coating operation is conducted in the right "straight line portion of the curve, there being no advantage to be gained in completing the coating operation with lower current densities since the straight line portion of my invention is always close to the conditions of maximum efficiency. In fact, it will be readily apparent that, in this respect, my invention has a great advantage over the prior practice. In that practice it has been necessary to deposit the greater part of the coating at comparatively low current densities, whereas, with my process, the entire coating can be given at high current densities.

As indicated above, my process is particularly adapted to be used with highlyacid electrolytes. Suitable electrolytes for such process may contain, for example, zinc from 50 to 110 grams per liter and sulfuric acid from 150 to 300 grams per liter. Temperatures should be at least as high as 49 C., and preferably higher. Temperatures up to the boiling point of the electrolyte may be employed. While temperatures in the neighbor- .hood of 100 C. are feasible from the electrochemical standpoint, I ordinarily prefer to use somewhat lower temperatures because of the dimculties which workmen must contend with when the electrolyte is very hot.

, In operating my process I employ current densities from 750 to 5000 amperes per square foot of cathode. surface but I ordinarily prefer to employ from 1600 to 3600 amperes per square foot of cathode surface.

As brought out by the article of Tainton and Leyson, cited above, it has been the prior practice to maintain the temperature of the electrolyte relatively low by providing means for accelerating the cooling of the electrolyte. In the practical carrying out of my process I find that it is usually necessary to provide means for heating the electrolyte, this being necessary not only because of the higher temperatures at which I work but because of the greater efilciencies of my process which result in less input of heat from the electrolytic circuit.

I have pointed out, earlier in this specification, that the curves'in the drawing are given merely as illustrative. I there indicated that the curves would-be somewhat different if the composition of the electrolyte or the temperature of operation were to be altered. By altering the composition of the electrolyte I have found it possible to produce a relationship between the efficiencies and current densities such that, if plotted, as is curve B of the drawing, would make the upper straight 35 line portion horizontal. In other words, when using current densities in excess of that required to produce maximum efliciency, further increase of current densities p'roduce unappreciable changes in efiiciency. I may accomplish this re- 40 sult by using an electrolyte containing zinc, 80 grams per litenand sulfuric acid, 220 grams per liter, and maintaining the temperature of the electrolyte at 75 C. By choice of suitable electrolyte composition I can even produce such a relationship between current densities and emciencies that the upper straight line portion of the curve would have a slight upward slope. Using an electrolyte temperature of 75 C., or higher temperature and an electrolyte containing 110 grams of zinc per liter and 222 grams of sulfuric acid per liter will accomplish this result.

In addition to the operative advantages pointed out above, and other advantages, I find that my use of high temperatures of electrolytes gives a better deposit of zinc, in that the zinc is of lighter color, somewhat more ductile and of finer surface texture.

Having thus described my invention what I claim as new and desire to secure by Letters Patent is:

1. In a process for the electrodeposition of zinc, the steps of electrolyzing a zinc containing solution having a sulphuric acid content from 150 grams to 300 grams per liter and a zinc content from 50 grams to 100 grams per liter at a current density at least as great as 750 amperes per square foot of cathode surface while maintaining such solution at a temperature at least as high as,

density between 750 and 5000 amperes per square 2. In a process for the electrodeposition of zinc,

foot of cathode surface while maintaining such solution at a temperature at least as high as 49 C.

3. In a process for the electrodeposition of zinc, the steps of electrolyzing a ,zinc containing solution having a sulphuric acid content from 150 grams to 300 grams per liter and a zinc content from 50 grams to 100 grams per liter at a current density from 1600 to 3600 amperes per square foot of cathode surface while maintaining such solution at a temperature of at least as high as 49' C.

4. In a process for the electrodeposition of zinc, the steps of electrolyzing a zinc containing solution having a sulphuric acid content from 150 grams to 300 grams per liter and a zinc content from 50 grams to 100 grams per liter at a current density from 750 to 5000 amperes per square foot of cathode surface while maintaining such solution at a temperature of from 49 C. up to the boiling point of the electrolyte.

5. In a process for the electrodeposition of zinc, the steps of electrolyzing a zinc containing solution having a sulphuric acid content from 150 grams to 300 grams per liter and a zinc content from 50 grams to 100 grams per liter at a current density of at least 750 amperes per square foot of cathode surface while maintaining such solution at a temperature from 60 C. to 102 C.

6. In a process for the electrodeposition of zinc; the steps of electrolyzing a zinc containing solution having a sulphuric acid content from 150 gramsto300 gramsperliterandazinc content from grams to 100 grams per liter at a current density of at least h ampere; per square foot of cathode surface while maintaining such solution at a temperature from C. ,to 102 C.

7. In a process for the electrodeposition of zinc, the steps of electrolyzing a zinc containing solution having a sulphuric acid content from 150 grams to 300 grams per liter and a zinc content from 50 grams to grams per liter at a current density of from 750 to 5000 amperes per square foot of cathode surface while maintaining such solution at a temperature from 60 C. to 102 C.

8. In a. process for the electrodeposition of zinc, the steps of electrolyzing a zinc containing solution having a sulphuric acid content from grams to 300 grams per liter and a zinc content from 50 grams to 100 grams per liter at a current density of from 1600 to 3600 amperes per square foot of cathode surface while maintaining such solution at a temperature from 60 C. to 102 C.

9. In a process for the electrodeposition of zinc, the steps of electrolyzing a zinc containing solution having a sulphuric acid content from 150 grams to 300 grams per liter and a zinc content from 50 grams to 100 grams per liter at a current density of from 1600 to 3600 amperes per square foot of cathode surface while maintaining such solution at a temperature from 60 C. to 102 C.

RICHARD M. WICK. 

